Method for laying tubular conduits

ABSTRACT

A method for laying a pipeline formed of joined rigid and flexible tubular conduits on an underwater surface, particularly an ocean bed, by using a floating support. The method includes the steps of: loading onto the floating support a length of flexible tubular conduit; laying from the support a length of rigid tubular conduit; connecting the ends of the two conduits together; lowering the conduits; and halting the lowering when the rigid conduit hits bottom, while the other end of the flexible conduit is still in the floating support.

Method for laying tubular conduits

The invention relates to a method for laying a pipeline formed of joinedrigid and flexible tubular conduits.

Rigid tubular conduits have the advantage of a relatively low linearcost. They have the disadvantage of being difficult to put into use. Inparticular, laying them is a long and costly operation. The laying ofrigid tubular conduits is carried out using a floating support, inparticular barges having low maneuverability. These barges only rarelycomprise devices for dynamic positioning. They are therefore forbiddento enter certain areas comprising platforms for drilling or fordeveloping oilfields. A method for laying such rigid tubular conduits isdescribed for example in GB-A-1204715.

Flexible tubular conduits have a higher cost per meter. However, theyare extremely quick to put into use. The applicant uses boats havingdynamic positioning in order to lay them. These ships do not usuallyallow rigid tubular conduits to be handled.

The method according to the present invention uses the respectiveadvantages of each type of conduit. The joining of flexible conduits andof rigid tubular conduits may be justified by the presence of anexisting network of rigid tubular conduits or by the performance ofwhich the devices for laying flexible conduits are capable.

According to the present invention, it is advantageous to use theequipment for laying rigid tubular conduits and in particular the bargesfor laying rigid tubular conduits, in order to lay a short length offlexible tubular conduit joined by one end to the terminal end of therigid tubular conduit. The length of flexible tubular conduit is greaterthan the depth of the water. Thus, once the end of the rigid tubularconduit rests on the bottom of the water, the second end of the flexibletubular conduit is still available on the barge for laying the rigidtubular conduit. This end is, for example for a subsequent resumption ofthe laying, transhipped on board a second floating support such as aship for laying flexible tubular conduits or abandoned on the bottom.The ship for laying flexible tubular conduits continues the laying ofthe flexible tubular conduit. It allows a quick and safe laying and inparticular allows congested sites to be approached, in so far as it is,advantageously, equipped with a device for dynamic positioning.

In one variant of the method according to the present invention, deviceswhich are compact and of low cost are used for carrying out the layingof the flexible tubular conduit from the barge for laying the rigidtubular conduits. Such an item of equipment comprises two pincers theopening and closing of which may be controlled; one pincer is fixed, onepincer is mobile. Such an item of equipment has the advantage of notcluttering the barge which is mainly intended for the laying of rigidtubular conduits. It has the disadvantage of being slow. However, in sofar as it is only used to lay short length of the flexible tubularconduit, this device only slightly slows down the operations of layingthe tubular conduits. However, it is of course understood that the useof the conventional tensioning means for laying flexible tubularconduits, for example of the type comprising a multi-caterpillar-trackedlinear winch, is not beyond the scope of the present invention.

At the end of the laying of the rigid tubular conduit it is necessary tocomply with the maximum mechanical stresses which such a conduit canwithstand. For example, fixing means are attached to the end of therigid tubular conduit. These fixing means comprise for example a flange.The fixing means enable the end of the rigid tubular conduit to besolidly fixed with the holding means intended to prevent the rigidtubular conduit from being subjected to bending of too small a radius ofcurvature. Steel cables for example are used.

The rigid tubular conduit is advantageously inclined for example at anangle between 10° and 30°, typically 20°, at the end of the barge. Asupporting device in the form of an inclined ramp called "stinger" isused for this purpose.

Once the flexible tubular conduit is joined to the end of the rigidtubular conduit, the simultaneous lowering of the terminal end of therigid tubular conduit and of the flexible tubular conduit is carriedout. For example, the cables holding the rigid tubular conduit areunwound with winches. During the lowering, the rigid tubular conduittakes up a shape having simple concavity, without any point ofinflection, of the catenary type. The flexible tubular conduit, held atone end by its joint with the rigid conduit, itself held by the cableswhich hold the rigid tubular conduit and having its other end held fromthe laying ship, bends under its own weight and substantially takes upthe shape of a catenary. The horizontal extension of the catenary may beregulated, and in particular reduced, by moving the laying barge.Advantageously, when the end of the rigid tubular conduit reaches thebottom, the cable is disconnected and raised to the surface.

In one embodiment, the flexible tubular conduit itself is used asholding means intended to ensure, that the rigid tubular conduit willnot be subjected to a bending of too small a radius of curvature whileit is being laid. In such a case, the flexible tubular conduit must beable to resist a tensile stress exerted by the apparent weight in waterof the rigid tubular conduit. This stress may for example be supportedby conventional flexible tubular conduits of large internal diameter forshallow waters. Specially reinforced flexible tubular conduits may alsobe used in order to be able to resist very large tensile stresses. Forexample, the number of tension-resisting reinforcing layers isincreased, or the section of the reinforcing wire used is increased. Itis of course understood that only the section intended to be laid on thebottom of the rigid tubular conduit needs to be reinforced. It will bepossible to carry on laying the flexible tubular conduit with sectionsof flexible tubular conduit which are not specially reinforced forresisting the tensile stress corresponding to the apparent weight inwater of the rigid tubular conduit.

The characteristics of the invention are the subject of claim 1.

The invention will be better understood by means of the followingdescription of the attached figures given as non-limiting examples, inwhich

FIGS. 1a and 1b are diagrams depicting the method according to thepresent invention using a submerged mobile pincer and a fixed pincer;

FIG. 2 is a diagram depicting the implementation of the method accordingto the present invention using two submerged pincers;

FIG. 3 is a diagram depicting the laying of flexible tubular conduitsjoined to a stretch of flexible tubular conduits laid by implementingthe method according to the present invention.

FIG. 4 is a diagram depicting an implementation variant of the methodaccording to the present invention in which the flexible tubular conduitacts as a means for holding the rigid tubular conduit.

FIG. 5 is a perspective diagram of the pincer for holding the conduits.

FIGS. 6a and 6b are top and front views, respectively, of the pincers ina first step of operation.

FIGS. 7a and 7b are top and front views of the pincers in a second stepof operation.

FIGS. 8a and 8b are top and front views, respectively, of the pincers inthird step of operation.

In FIGS. 1 to 4, the same reference numerals have been used to designatethe same elements.

In FIG. 1, an example of laying a pipeline made up of a rigid tubularconduit 5 and of a flexible tubular conduit 15 joined to each other. Thelength of the flexible tubular conduit 15 is slightly greater than thedepth P of the bottom 7. The length of the flexible tubular conduit 15takes into account the length necessary for this conduit to form acatenary during the laying of the end of the rigid tubular conduit. Oncethe end of the rigid conduit has been laid on the bottom by means of afirst floating support, such as a barge 1 for laying rigid tubularconduits, the laying of flexible tubular conduits 15 will advantageouslybe carried on by means of a ship specialised in the laying of flexibletubular conduits.

The rigid tubular conduit 5 is held by a holding means 12, for example aclamping pincer connected by a cable 16 to a winch 14. The holding means12 is placed at the connection between the rigid tubular conduit 5 andthe flexible tubular conduit 15. The flexible tubular conduit 15 isheld, on the one hand, by the end of the rigid conduit 5, and on theother hand by a mobile pincer 2 and/or by a fixed pincer 3. The flexibletubular conduit 15 substantially takes up the shape of a catenary underthe effect of its own weight.

Once the end of the rigid tubular conduit has left the floating support,for example the barge 1, the laying boat or ship, it is possible to"move back" the floating support in the direction opposite to the arrow11 so as to reduce the horizontal distance of the suspended rigidconduit portion in order to pass from an S-configuration to a simpleconcave catenary without a point of inflection.

At the joint with the rigid tubular conduit, the flexible tubularconduit has a curve of inverted concavity (swan neck shape).

The catenary formed by the flexible tubular conduit is progressivelydeformed as the lowering operation takes place. When the flexibletubular conduit touches the bottom, it forms a catenary which is joinedtangentially to the end of the rigid tubular conduit.

In FIG. 1a, the mobile pincer 2 has been depicted in its high position.Once the mobile pincer 2 is clamped around the flexible tubular conduit15, the fixed pincer 3 may be unclamped. The simultaneous lowering ofthe rigid tubular conduit 5 is carried out by unwinding the cable 16from the winch 14, and the lowering of the tubular conduit 15 is carriedout by unwinding a second cable 29 from the winch 4.

In FIG. 1b, the pincer 2 has been depicted in its low position. At thispoint, the fixed pincer 3 is clamped around the flexible tubular conduit15 and the mobile pincer 2 is raised. In the example depicted in FIG. 1,the fixed pincer 3 is located near the bow (according to the directionof the arrow 11) of the boat 1. In practice, this is not at allnecessary. As the flexible tubular conduit 15 is progressively lowered,it is unwound, for example by storage means of the "basket" or "cabledrum" type (not shown in FIG. 1). When the mobile pincer 2 has reachedits low position, the fixed pincer 3 is clamped. The pincer 2 is thenopened and by winding the cable 29 onto the winch 4, the mobile pincer 2is raised. The beginning of the cycle depicted in FIG. 1a is arrived atonce again.

Advantageously, when the pincer 12 has reached the bottom 7, it isreleased and raised by the winch 14. The pincers 2 and 3 are illustratedin FIGS. 5-8. The pincers 2 and 3 each include a main frame 31, twomovable key-shaped jaws 32, and door 33, and maneuvering cables orhydraulic jacks 34. A flexible conduit 35 is gripped by the jaws 32.Referring to FIG. 6, the jaws 32 are in an open position allowing thepincer to be placed around a conduit 35. In FIG. 7, the door 33 isclosed and the jaws 32 remain in their open position allowingtranslation of the pincer along the conduit 35. In FIG. 8, the jaws 32have closed about the conduit 35. The door 33 is not shown in FIG. 6asince the door is open, however, the door 33 is shown in dotted line andFIG. 7a and 8a.

In FIG. 2, the implementation of the method according to the presentinvention using a plurality of pincers placed on the deck of the barge 1may be seen. In the example depicted, two pincers, one being fixed 3,the other mobile 2, fixed on the deck of the barge 1, let goalternately, enable the flexible tubular conduit 15 to be laid.Simultaneously, the unwinding of the cable 16 from the winch 14 enablesthe rigid tubular conduit 5 to be laid. In the example depicted in FIG.2, the flexible tubular conduit 15 is unwound from a storage means 19.

In one variant of the method according to the invention, the second endof the flexible tubular conduit 15 is abandoned on the bottom.Advantageously, such an end abandoned on the bottom will comprise meansfacilitating its recovery such as for example connection devices and/orlocating means. The locating means will comprise for example a sonarbeacon. For example, a passive sonar beacon will be used forabandonments of long duration, and an active sonar beacon will be usedfor abandonments of short duration.

In one particularly advantageous variant of the method according to theinvention, once the end of the rigid tubular conduit rests on thebottom, the second end of the flexible tubular conduit 15 lying ready onthe floating support 1 is transhipped onto a second laying support suchas a ship specialized in the laying of flexible tubular conduits, forexample as depicted in FIG. 3, a quick-laying ship 24 comprising means21 for dynamic positioning. The continuation of the operations of layingflexible tubular conduits will be carried out using this ship (asdepicted in FIG. 3). In order to tranship the flexible tubular conduitfrom the first floating support 1, for example from a barge, to a secondfloating support, for example a ship 24, a steel cable is firstadvantageously passed between the barge 1 and the ship 24, the end ofthe flexible tubular conduit to be transhipped being solidly fixed tothe end of this cable. This solid fixing is carried out for example bymeans of a flange. A traction on the cable, for example by means of awinch installed on the ship 24, enables the end of the flexible tubularconduit to be brought on board the second floating support.

Such a ship will be able to lay a great length of flexible tubularconduit 15' joined to the tubular conduit 15 laid by the barge 1 and/orapproach a drilling or production platform 25 without there being anydanger of colliding with this platform.

In FIG. 4, an example of implementation of a laying method according tothe present invention may be seen in which the flexible tubular conduit15 holds the rigid tubular conduit 5. The conduits 5 and 15 are solidlyfixed to each other by a connection means 28, for example comprisingflanges. For example a first flange is welded to the end of the rigidconduit 5. A second flange is fixed to the end of the joining piece, notshown, fixed to the flexible tubular conduit 15.

We claim:
 1. A method for laying a pipeline formed of joined flexible tubular conduits and rigid tubular conduits, onto a bottom located at a depth P, in particular an ocean bed, which comprises the steps consisting of:loading a length, greater than the depth P, of flexible tubular conduit onto a first floating support; laying a length of rigid tubular conduit (5), the terminal end of which is held by said first floating support, onto the bottom, from a first floating support (1); connecting a first end of said flexible tubular conduit (15) having a length greater than the depth of the water P to the free terminal end of said rigid tubular conduit (5); simultaneously lowering the terminal end of the rigid tubular conduit (5) and the flexible tubular conduit (15); continuing the lowering until the first end of the flexible tubular conduit connected to the terminal end of the rigid tubular conduit is located on the bottom (7) and whereat the second end of the flexible conduit (15) is still located on board the first floating support (1); transferring said second end of the flexible tubular conduit (15) onto a second floating support (24), to connect the end of another flexible tubular conduit (15') to said second end of the flexible tubular conduit (15) and laying the section of pipeline comprising said flexible tubular conduits (15, 15') onto the bottom (7).
 2. The method as claimed in claim 1, wherein the step consisting of transferring said second end of the flexible tubular conduit (15) includes the transhipping of said end from the first floating support (1) to the second floating support (24).
 3. The method as claimed in claim 1, wherein the step consisting of transferring said second end of the flexible tubular conduit (15) is implemented after a step consisting in abandoning the flexible tubular conduit (15) from said first floating support, said transfer step comprising a step consisting in recovering, by means of the second floating support (24), the flexible tubular conduit (15) abandoned by said first floating support (1).
 4. The method as claimed in claim 1, wherein said first floating support (1) is a barge for laying rigid tubular conduits (5).
 5. The method as claimed in claim 4, wherein the barge for laying rigid tubular conduits (5) is a barge for S-shaped laying comprising a ramp (10) for supporting the rigid tubular conduit (5).
 6. The method as claimed in claim 1, wherein said second floating support (24) is a specialized ship for laying flexible tubular conduits.
 7. The method as claimed in claim 1, which comprises a step consisting of fixing holding means, such as a traction cable (16), to the end of the rigid tubular conduits (5) via fixing means (12), the holding means being operated from the first floating support (1).
 8. The method as claimed in claim 1, wherein the flexible tubular conduit (15) holds the rigid tubular conduit (5) during the laying operation. 